1. Field of the Invention
The present disclosure relates generally to inertial igniters and more particularly to inertial igniters for thermal batteries or other pyrotechnic type initiated devices for munitions such as gun fired or mortar rounds or rockets with safety arm.
2. Prior Art
Thermal batteries represent a class of reserve batteries that operate at high temperature. Unlike liquid reserve batteries, in thermal batteries the electrolyte is already in the cells and therefore does not require a distribution mechanism such as spinning. The electrolyte is dry, solid and non-conductive, thereby leaving the battery in a non-operational and inert condition. These batteries incorporate pyrotechnic heat sources to melt the electrolyte just prior to use in order to make them electrically conductive and thereby making the battery active. The most common internal pyrotechnic is a blend of Fe and KClO4. Thermal batteries utilize a molten salt to serve as the electrolyte upon activation. The electrolytes are usually mixtures of alkali-halide salts and are used with the Li(Si)/FeS2 or Li(Si)/CoS2 couples. Some batteries also employ anodes of Li(Al) in place of the Li(Si) anodes. Insulation and internal heat sinks are used to maintain the electrolyte in its molten and conductive condition during the time of use. Reserve batteries are inactive and inert when manufactured and become active and begin to produce power only when they are activated.
Thermal batteries have long been used in munitions and other similar applications to provide a relatively large amount of power during a relatively short period of time, mainly during the munitions flight. Thermal batteries have high power density and can provide a large amount of power as long as the electrolyte of the thermal battery stays liquid, thereby conductive. The process of manufacturing thermal batteries is highly labor intensive and requires relatively expensive facilities. Fabrication usually involves costly batch processes, including pressing electrodes and electrolytes into rigid wafers, and assembling batteries by hand. The batteries are encased in a hermetically-sealed metal container that is usually cylindrical in shape. Thermal batteries, however, have the advantage of very long shelf life of up to 20 years that is required for munitions applications.
Thermal batteries generally use some type of igniter (initiator) to provide a controlled pyrotechnic reaction to produce output gas, flame or hot particles to ignite the heating elements of the thermal battery.
There are currently two distinct classes of igniters that are available for use in thermal batteries. The first class of igniter operates based on electrical energy. Such electrical igniters, however, require electrical energy, thereby requiring an onboard battery or other power sources with related shelf life and/or complexity and volume requirements to operate and initiate the thermal battery. The second class of igniters, commonly called “inertial igniters”, operates based on the firing acceleration. These (mechanical) inertial igniters do not require onboard batteries for their operation and are thereby often used in high-G munitions applications such as in gun-fired munitions and mortars.
In munitions, the need to differentiate accidental and initiation accelerations, i.e., the so-called no-fire and all-fire (set-back) accelerations, respectively, by the resulting impulse level of the event necessitates the employment of a safety system which is capable of allowing initiation of the igniter only during high total impulse levels. In mechanical inertial igniters, the safety mechanism can be thought of as a mechanical delay mechanism, after which a separate initiation system is actuated or released to provide ignition of the pyrotechnics. Such mechanical inertial igniters that combines such a safety system with an impact based initiation system of different types are described, for example, in U.S. Pat. Nos. 7,437,995; 7,587,979; 7,587,980; 7,832,335; 8,042,469; and 8,061,271; U.S. Patent Application Publication Nos. 2010/0307362; 2011/0171511; 2012/0180680; 2012/0180681; 2012/0180682; 2012/0205225 and 2012/0210896 and U.S. patent application Ser. Nos. 12/794,763; 12/955,876 and 13/180,469; the disclosures or each of which are incorporated by reference.
Inertia-based (mechanical) igniters must therefore comprise two components so that together they provide the aforementioned mechanical safety (delay mechanism) and to provide the required striking action to achieve ignition of the pyrotechnic elements. The function of the safety system is to fix the striker in position until a specified acceleration time profile actuates the safety system and releases the striker, allowing it to accelerate toward its target under the influence of the remaining portion of the specified acceleration time profile. The ignition itself may take place as a result of striker impact, or simply contact or proximity. For example, the striker may be akin to a firing pin and the target akin to a standard percussion cap primer. Alternately, the striker-target pair may bring together one or more chemical compounds whose combination with or without impact will set off a reaction resulting in the desired ignition.
As an example, the isometric cross-sectional view of an inertial igniter described in U.S. Patent Application Publication No. 2011/0171511 is shown in FIG. 1, referred to generally with reference numeral 200. The full isometric view of the inertial igniter 200 is shown in FIG. 2. The inertial igniter 200 is constructed with igniter body 201, consisting of a base 202 and at least three posts 203. The base 202 and the at least three posts 203, can be integrally formed as a single piece but may also be constructed as separate pieces and joined together, for example by welding or press fitting or other methods commonly used in the art. The base 202 of the housing can also be provided with at least one opening 204 (with a corresponding opening(s) in the thermal battery—not shown) to allow ignited sparks and fire to exit the inertial igniter and enter into the thermal battery positioned under the inertial igniter 200 upon initiation of the inertial igniter pyrotechnics 215, or initiation of a percussion cap primer when used in place of the pyrotechnics.
A striker mass 205 is shown in its locked position in FIG. 1. The striker mass 205 is provided with guides for the posts 203, such as vertical surfaces 206, that are used to engage the corresponding (inner) surfaces of the posts 203 and serve as guides to allow the striker mass 205 to ride down along the length of the posts 203 without rotation with an essentially pure up and down translational motion.
In its illustrated position in FIGS. 1 and 2, the striker mass 205 is locked in its axial position to the posts 203 by at least one setback locking ball 207. The setback locking ball 207 locks the striker mass 205 to the posts 203 of the inertial igniter body 201 through the holes 208 provided in the posts 203 and a concave portion such as a dimple (or groove) 209 on the striker mass 205 as shown in FIG. 1. A setback spring 210, which is preferably in compression, is also provided around but close to the posts 203 as shown in FIGS. 1 and 2. In the configuration shown in FIG. 1, the locking balls 207 are prevented from moving away from their aforementioned locking position by the collar 211. The setback spring 210 can be a wave spring with rectangular cross-section. The rectangular cross-section eliminates the need to fix or otherwise retain the striker spring 210 to the collar 211, which is an expensive process; the flat coil spring surfaces minimizes the chances of coils slipping laterally (perpendicular to the direction of acceleration 218), which can cause jamming and prevent the release of the striker mass 205 (preventing the collar to move down enough to release the locking balls). Furthermore, wave springs generate friction between the waves at contact points along the spring wire, thereby reducing the chances for the collar 211 to rapidly bounce back up and preventing the striker mass 205 from being released.
The collar 211 is preferably provided with partial guide 212 (“pocket”), which are open on the top as indicated by the numeral 213. The guide 212 may be provided only at the location of the locking balls 207 as shown in FIGS. 1 and 2, or may be provided as an internal surface over the entire inner surface of the collar 211. The advantage of providing local guides 212 is that it results in a significantly larger surface contact between the collar 211 and the outer surfaces of the posts 203, thereby allowing for smoother movement of the collar 211 up and down along the length of the posts 203. In addition, they prevent the collar 211 from rotating relative to the inertial igniter body 201 and make the collar stronger.
The collar 211 rides up and down on the posts 203 as can be seen in FIGS. 1 and 2, but is biased to stay in its upper most position as shown in FIGS. 1 and 2 by the setback spring 210. The guides 212 are provided with bottom ends 214, so that when the inertial igniter is assembled as shown in FIGS. 1 and 2, the setback spring 210 which is biased (preloaded) to push the collar 211 upward away from the igniter base 202, would “lock” the collar 211 in its uppermost position against the locking balls 207. As a result, the assembled inertial igniter 200 stays in its assembled state and would not require a top cap to prevent the collar 211 from being pushed up and allowing the locking balls 207 from moving out and releasing the striker mass 205.
In the embodiment 200, a one part pyrotechnics compound 215 (such as lead styphnate or other similar compound) can be used as shown in FIG. 1. The striker mass can be provided with a relatively sharp tip 216 and the igniter base surface 202 is provided with a protruding tip 217 which is covered with the pyrotechnics compound 215, such that as the striker mass is released during an all-fire event and is accelerated down (opposite to the arrow 218 illustrated in FIG. 1), impact occurs mostly between the surfaces of the tips 216 and 217, thereby pinching the pyrotechnics compound 215, thereby providing the means to obtain a reliable initiation of the pyrotechnics compound 215. Alternatively, a two-part pyrotechnics, e.g., potassium chlorate (first part), can be provided on the base 202 over the exit hole 204 and a second part consisting of red phosphorus can be provided on the lower surface of the striker mass surface 205 over the area of the sharp tip 216.
Alternatively, instead of using the pyrotechnics compound 215, FIG. 1, a percussion cap primer or the like can be used. A striker tip is generally provided at the tip 216 of the striker mass 205 to facilitate initiation upon impact.
The basic operation of the embodiment 200 of the inertial igniter of FIGS. 1 and 2 is as follows. If the inertial igniter is subjected to any non-trivial acceleration in the axial direction 218 which can cause the collar 211 to overcome the resisting force of the setback spring 210 will initiate and sustain some downward motion of the collar 211. The force due to the acceleration on the striker mass 205 is supported at the dimples 209 by the locking balls 207 which are constrained inside the holes 208 in the posts 203. If an acceleration amplitude and duration in the axial direction 218 imparts a sufficient impulse (i.e., an impulse greater than a predetermined threshold) to the collar 211, it will translate down along the axis of the assembly until the setback locking balls 205 are no longer constrained to engage the striker mass 205 to the posts 203. If the acceleration event is not sufficient to provide this motion (i.e., the acceleration time profile provides less impulse than the predetermined threshold), the collar 211 will return to its start (top) position under the force of the setback spring 210.
Assuming that the acceleration time profile was at or above the specified “all-fire” profile, the collar 211 will have translated down past the locking balls 207, allowing the striker mass 205 to accelerate down towards the base 202. In such a situation, since the locking balls 207 are no longer constrained by the collar 211, the downward force that the striker mass 205 has been exerting on the locking balls 207 will force the locking balls 207 to move outward in the radial direction. Once the locking balls 207 are out of the way of the dimples 209, the downward motion of the striker mass 205 is no longer impeded. As a result, the striker mass 205 is accelerated downward, causing the tip 216 of the striker mass 205 to strike the pyrotechnic compound 215 on the surface of the protrusion 217 with the requisite energy to initiate ignition.
In the embodiment 200 of the inertial igniter shown in FIGS. 1 and 2, the setback spring 210 is illustrated as a helical wave spring type fabricated with rectangular cross-sectional wires (such as the ones manufactured by Smalley Steel Ring Company of Lake Zurich, Ill.). The use of such rectangular cross-section wave springs or the like has the following significant advantages over helical springs that are constructed with wires with circular cross-sections. Firstly and most importantly, as the spring is compressed and nears its “solid” length, the flat surfaces of the rectangular cross-section wires come in contact and generate minimal lateral forces that would otherwise tend to force one coil to move laterally relative to the other coils as is usually the case when the wires are circular in cross-section. Lateral movement of the coils can, in general, interfere with the proper operation of the inertial igniter since it could, for example jam a coil to the outer housing of the inertial igniter (not shown), which is usually desired to house the igniter 200 or the like with minimal clearance to minimize the total volume of the inertial igniter. In addition, the laterally moving coils could also jam against the posts 203 thereby further interfering with the proper operation of the inertial igniter. The use of such wave springs with rectangular cross-section eliminates such lateral movement and therefore significantly increases the reliability of the inertial igniter and also significantly increases the repeatability of the initiation for a specified all-fire condition. The second advantage of the use of the aforementioned wave springs with rectangular cross-section, particularly since the wires can and are usually made thin in thickness and relatively wide, the solid length of the resulting wave spring can be made to be significantly less than an equivalent regular helical spring with circular cross-section. As a result, the total height of the resulting inertial igniter can be reduced. Thirdly, since the coil waves are in contact with each other at certain points along their lengths and as the spring is compressed, the length of each wave is slightly increased, therefore during the spring compression the friction forces at these contact points do a certain amount of work and thereby absorb a certain amount of energy. The presence of such friction forces ensures that the firing acceleration and very rapid compression of the spring would to a lesser amount tend to “bounce” the collar 211 back up and thereby increasing the possibility that it would interfere with the exit of the locking balls from the dimples 209 of the striker mass 205 and the release of the striker mass 205. The above characteristic of the wave springs with rectangular cross-section therefore also significantly enhances the performance and reliability of the inertial igniter 200 while at the same time allowing its height (and total volume) to be reduced.
In the prior art inertial igniters similar the one illustrated in FIGS. 1 and 2, by varying the mass of the striker 205, the mass of the collar 211, the spring rate of the setback spring 210, the distance that the collar 211 has to travel downward to release the locking balls 207 and thereby release the striker mass 205, and the distance between the tip 216 of the striker mass 205 and the pyrotechnic compound 215 (and the tip of the protrusion 217), the designer of the disclosed inertial igniter 200 can match the all-fire and no-fire impulse level requirements for various applications as well as the safety (delay or dwell action) protection against accidental dropping of the inertial igniter and/or the munitions or the like within which it is assembled.
Briefly, the safety system parameters, i.e., the mass of the collar 211, the spring rate of the setback spring 210 and the dwell stroke (the distance that the collar 210 has to travel downward to release the locking balls 207 and thereby release the striker mass 205) must be tuned to provide the required actuation performance characteristics. Similarly, to provide the requisite impact energy, the mass of the striker 205 and the aforementioned separation distance between the tip 216 of the striker mass and the pyrotechnic compound 215 (and the tip of the protrusion 217) must work together to provide the specified impact energy to initiate the pyrotechnic compound when subjected to the remaining portion of the prescribed initiation acceleration profile after the safety system has been actuated.
The inertial igniters of the type described above have been shown to be capable of being miniaturized and provide highly reliable means of initiating thermal batteries or the like. In certain applications, particularly in applications in which the firing (setback) acceleration for initiating the thermal battery is relatively low and/or its duration is relatively short, then the acceleration levels that the inertial igniter could accidentally be subjected to might be even higher than the intended all-fire (setback) acceleration and/or duration. This would also be the case if the munitions in which the inertial igniter is used are required to survive shock loading due to drops from relatively high heights of the order of 40 feet or nearby explosions without the thermal battery (inertial igniter) initiation. In such situations, the aforementioned safety mechanisms would not prevent inertial igniter initiation since shock impulse that could be experienced by the inertial igniter could be higher than that of the firing setback. In such applications, it is highly desirable to provide the inertial igniter integrated thermal battery with safing arm (pin) that has to be removed (actuated or inserted or the like) to make the inertial igniter operational in response to the prescribed all-fire shock profile.